Does partial tracing make us see things that do not occur?

[Heidi]

Hi Pfs
Partial tracing maps what occurs in a big Hilbert space toward a smaller one. We have to use it when degrees of freedom are physically unobservable or when we have only a coarse grained view of the environment. it is like in Flatland , where the two dimensional inhabitants has no access to the third dimension. non they only see the shadows of what is above. for them non parallel lines always have an intersection point even if this point does not exist above them. They see this point !
Are there also for us things that we see and measure which are like these intersection points ( due to partial tracing)?

 

[Heidi]

we know that particles do not follow trajectories but we see the pictures of charged particles in a magnetic field. Is it not the same for the notion of event? is the emition of a photon an event like those in our life?

 

[vanhees71]

I guess you mean the tracks charged particles make in a cathode ray tube showing "classical trajectories" under the influence of electric and/or magnetic fields and which were used in the early 1900ds to check Einstein's special theory of relativity (e.g., by Kaufmann) vs. other electron-theoretical models (most famously Abraham's).

This is of course no contradiction to quantum theory and well understood since 1929:

N. Mott, The Wave Mechanics of alpha-Ray Tracks,
Proceedings of the Royal Society of London. Series A 126, 79
(1929), https://doi.org/10.1098/rspa.1929.0205

 

[Demystifier]

non they only see the shadows of what is above. for them non parallel lines always have an intersection point even if this point does not exist above them. They see this point !

I don't think that it's a good metaphor for partial tracing. Partial tracing is just a quantum analog of marginal probability computation.
https://en.wikipedia.org/wiki/Marginal_distribution

 

[Demystifier]

Are there also for us things that we see and measure which are like these intersection points ( due to partial tracing)?

An example of measurable quantity calculated with the help of partial tracing is entropy.

 

[Demystifier]

Does partial tracing make us see things that do not occur?

It's actually the opposite, often partial tracing makes us not see things that occur.

 

[Heidi]

Yes partial tracing can give the entropy. It shows that at the beginning the entropy of the universe was very very low. it is what we calculate and "see". but is it an objective thing?
Rovelli has doubts. maybe it looks like that because WE are now in a peculiar region or we are peculiar observers. this low entropy correspond to our coarse grained point of view.

what are for you the open problems in QM?

 

[Demystifier]

Yes partial tracing can give the entropy. It shows that at the beginning the entropy of the universe was very very low. it is what we calculate and "see". but is it an objective thing?
Rovelli has doubts. maybe it looks like that because WE are now in a peculiar region or we are peculiar observers. this low entropy correspond to our coarse grained point of view.

Yes entropy is objective, and yes entropy depends on our coarse graining. I don't see a contradiction.

what are for you the open problems in QM?

The measurement problem and (closely related) the ontology problem.